A typical swimming pool is an upwardly-open cup-like structure for containing pool water. The pressure of the contained water tends to downwardly and outwardly drive the pool. On the other hand, in the case of in-ground pools, ground water pressure acts on the pool tending to drive it inwardly and upwardly.
Rising ground water is problematic for in-ground pools. When the ground water pressure exceeds the pool water pressure, the pool can bulge and/or crack and/or float out of the ground in the manner of a boat. Bulging and/or cracking are the more common failure modes in fibreglass pools whereas floating is the more common failure mode in concrete pools.
Swimming pools now incorporate pressure relief valves with the intent of addressing these problems. These valves are also known as hydrostatic relief valves.
Certain fibreglass pools incorporate a hydrostatic relief valve incorporating a vertically oriented cylindrical tubular body having a diameter of about 50 mm. The body includes a lower externally-threaded portion by which the body is threadingly engaged with the pool floor.
The cylindrical body is capped by a concentrically-mounted horizontal disc, mounted to move vertically. The disc is constrained by a spring mounting which downwardly drives the disc to compress an O-ring seal between the disc and the cylindrical body. Thus a sealing arrangement is formed so as to block the vertical flow path through the centre of the cylindrical body.
This sealing arrangement serves to prevent the pool water escaping from the pool via the valve. When the ground water pressure sufficiently exceeds the pool water pressure, the ground water acting on the disc upwardly drives the disc against its spring mounting. The disc is lifted about 10 mm from the cylindrical body to define a cylindrical gap through which the ground water may flow radially outwards into the pool.
Some concrete swimming pools incorporate a hydrostatic relief valve in the form of a “grease plate”. A grease plate is an arrangement including a circular aperture formed in the floor of the pool. This aperture has a diameter in the vicinity of 160 mm and an upwardly-diverging conical wall. A thin-walled dinner plate-like disc fits within the aperture and has a conical rim complementary to the conical wall of the aperture. The top of the disc is typically filled with concrete (and tile, etc) to match the finish of the pool floor. The complementary conical surfaces are coated in grease so that they may co-operate to form an effective seal.
Despite the use of such valves, pools continue to fail. The present inventors have recognised that such failures may be attributed to existing relief valves:                1 allowing ground water pressure to build to destructive levels before the valve opens; and/or        2. providing inadequate pressure relief even when opened.        
Australian Standard AS/NZS 1839:1994 suggests that pressure relief valves should “operate to relieve external water pressure so that the maximum pressure differential across the valve cannot exceed 30 mm of water”. The inventors' own investigations have shown that various existing valves do not even open at this pressure differential.
The inventors have also recognised that existing valves leak and so sometimes require a diver to enter the pool to service the valve. Indeed diving is typically required whenever a grease plate is actuated.
Accordingly the invention aims to provide an improved valve for relieving ground water pressure on a swimming pool, or at least to provide an alternative valve in the marketplace.
It is not admitted that any of the information in this patent specification is common general knowledge, or that the person skilled in the art could be reasonably expected to ascertain or understand it, regard it as relevant or combine it in any way at the priority date.